New to the site, somewhat new to radio. SWR question.

[Lo/Dx Frontier]

Hello! New guy here. Had a couple of CB’s back in the 90’s but not for very long. Looking to get back into it again but this time I want to learn the ins and outs of it instead of just connect a radio to an antenna and set SWR and then just talk. So I’ve been reading and watching videos a lot lately on several forms of radio, CB, GMRS, MURS, marine VHF, and some HAM.

This brings me to my first question on SWR. From what I understand, any antenna length is set based on what frequency or band of frequencies you wish to receive and transmit on. Longer wavelengths require longer antennas it seems. So does this mean that once I get an antenna set to the right length for an SWR down near 1:1, on any of the radio types I mentioned above, if I swap radios I shouldn’t need to reset the antenna SWR again should I? I’m only asking for basic knowledge and understanding. I have yet to choose a radio and antenna. I’m just gathering knowledge as of now.

 

[Crawdad]

Hello New Guy Here and welcome!
You can swap radios once you have an antenna tuned to a certain frequency band but you won't be able to swap frequency bands, if that makes sense, i.e. you can't jump from 11m to GMRS with the same antenna. Unless that is you have antenna designed to give you a usable SWR on multiple bands.

You might think of studying for and getting an Amateur Radio (Ham) license, it's a good way to learn as you play.

73

 

[midnight special]

as long as ya stay on the same band as the antenna is set for ya ok

 

[fourstringburn]

Welcome to the Forum!

Yes, your answers are correct.

When you change radio's and you see a significant difference in SWR, then the radio's SWR meter(s) are suspect because your antenna system didn't change.

Always best to use a quality external meter to check and tune your antennas and compare the difference with any radio that your using. Most built in meters on CB and even many expensive Export radio's meters aren't that precise but should be in the ball park enough to use as a guide line.

Many of us that build are own antennas or want precise testing equipment use antenna analyzers. This makes checking and tuning quick and easy once you learn to use them.

When you tune the antenna, the most common advice given is to check tuning between channels 1 and 40. While this isn't necessarily wrong, it's just gives a balanced compromise on the band.

Best way is to tune to the lowest SWR where you operate and let the antennas bandwidth cover the rest of the channels. Depending on the antenna type, design, and mounting, You can have a near flat SWR on your most used frequency and up to +/- 3 channels before the SWR curve will start to rise.

On the CB band, if you tuned near flat on channel 19, your SWR will rise on 1 and 40 and might be out of balance between the two but overall your antenna was tuned for the best resonance so no need to worry if the SWR on the band edges climb up to 1.9 as it won't hurt anything or make a difference on the other end of the receiving station.

Hope that information helps and good luck with your setup!

 

[Lo/Dx Frontier]

Thanks for so many answers so fast! So this confirms that I understood it correctly. I was getting the sense that the antenna length and frequency go hand in hand and once SWR is set then the antenna is good for that frequency regardless of the radio, so long as its the same kind of radio (set up antenna for 11 meter band and then swap CB radio for CB radio, and so on).

My next question is does power effect SWR? Legally all CB’s are 4 watts. By the way, is that just 4 watts at dead key, and then the wattage swings up as you speak? And what if a person amped up the wattage output of a radio, does the SWR need to be reset again? Basically is SWR frequency related only, or is it frequency related at a certain power and then changes as power changes?

Also, I’ve read that the ideal antenna length for CB is about 32 feet based on a 32 foot wavelength. Does this mean that all antennas can be set to match the wavelength of whatever frequency you want to receive and transmit on? GMRS at 462MHz is up above the 70cm band (about 62cm) so I’m getting the idea that the perfect antenna there would be about 2 feet long. Is that how it works? I understand that some antennas are shorter based on 1/4, 1/2, and 5/8 wavelengths (so I’ve read for CB).

 

[Crawdad]

Just to give you a starting place, here is a 1/4 wavelength Vertical Ground Plane for 11m:

 

[Crawdad]

A Half-wavelength Dipole like this:

Can be made from wire and calculated using:
468/frequency in MHz for both halves or "poles"
468/27.185=17.2 ft
or
234/frequency in MHz for one half or "pole"
234/27.185=8.6 ft

 

[Lo/Dx Frontier]

A Half-wavelength Dipole like this:
View attachment 63998
Can be made from wire and calculated using:
468/frequency in MHz for both halves or "poles"
468/27.185=17.2 ft
or
234/frequency in MHz for one half or "pole"
234/27.185=8.6 ft

Thank you for the diagrams. This one in particular interests me a lot. I’ve read about dipole antennas some but I’m unclear when you would want to use this over a vertical antenna. Maybe just for convenience of size? I could see this up on the roof of a house and keeping it sort of low profile as opposed to one sticking straight up. What is the advantage of this over the other?

 

[Crawdad]

What is the advantage of this over the other?

This is just my opinion, others will disagree:

Half-wave Center-fed Dipoles
Pros:

• Easy to design, install and tune to the band of your choice.
• Inexpensive to make, repair, replace. How expensive is 16' of wire? (for 11m).
• Can be bent (under certain parameters) to fit individual situations.
• A great learning tool.
• Work quite well, despite the popular myths.
• Installed horizontally they're excellent for DX.
• Used by Amateurs all over the world to talk all over the world on many frequency bands.
• Balanced, efficient antennas when installed and tuned correctly. No groundplane/counterpoise required.
• Can be directly fed with 50ohm coax, no lossy matching coils required.

Cons:

• Horizontal polarization may not work great for talking locally to stations with vertically polarized antennas.
• Can be tricky to tune and use mounted vertically.
• Don't offer as much gain as a few designs of verticals or most Beam antennas.

Lots of great info on dipoles here:
https://www.qsl.net/kk4obi/

Hope this helps,
73

 

[Lo/Dx Frontier]

Wow. I don’t even have a radio yet and I’m already envisioning a half wave dipole up in the attic and connected to a radio in one of our spare rooms here for talking skip. Then a second radio connected to a vertical antenna for local chatter. LOL!

 

[Crawdad]

Wow. I don’t even have a radio yet and I’m already envisioning a half wave dipole up in the attic and connected to a radio in one of our spare rooms here for talking skip. Then a second radio connected to a vertical antenna for local chatter. LOL!

You may have caught the RF virus.............